Method and device in a communication network

ABSTRACT

A method of operating a base station such that the method comprises determining whether there are any mobile devices that are not associated with the base station that require protection from interference caused by downlink transmissions of the base station and setting a maximum permitted transmission power for the base station based on the result of the step of determining. A base stations operating according to this method is also disclosed.

1. PRIORITY CLAIM

This application is a continuation of and claims priority to U.S. patentapplication Ser. No. 12/794,128 filed on Jun. 4, 2010 issuing as U.S.Pat. No. 8,463,312, which claims priority to Great Britain ApplicationNo. 0909649.6 filed on Jun. 5, 2009.

2. FIELD OF THE INVENTION

The invention relates to communication networks that include basestations, and in particular to a method for controlling a maximumpermitted transmission power for downlink transmissions from basestations, and a base station configured to perform the method.

3. BACKGROUND TO THE INVENTION

Femtocell base stations in a Long Term Evolution (LTE) communicationnetwork (otherwise known as Home evolved Node Bs—HeNBs—or Enterpriseevolved Node Bs—EeNBs) are small, low-power, indoor cellular basestations for residential or business use. They provide better networkcoverage and capacity than that available in such environments from theoverlying macrocellular LTE network. In addition, femtocell basestations use a broadband connection to receive data from and send databack to the operator's network (known as “backhaul”).

As femtocell base stations can make use of the same frequencies asmacrocell base stations in the macrocellular network, and as they arelocated within the coverage area of one or more macrocell base stationsin the macrocellular network, it is necessary to ensure that downlinktransmissions from the femtocell base station to mobile devices(otherwise known as User Equipments—UEs) using the femtocell basestation do not interfere substantially with downlink transmissions frommacrocell base stations to mobile devices using the macrocell basestations.

Typically, this interference is mitigated by placing a cap on the powerthat the femtocell base station can use to transmit signals to mobiledevices. The cap on the power can be set such that, at a specifiedpathloss from the femtocell base station (for example 80 dB), a signalreceived by a mobile device from a macrocell base station would meet aspecified quality level (for example a target signal to interferenceplus noise ratio—SINR). The determination of the cap is subject to aminimum and maximum power restriction on the transmission power of thefemtocell base station, for example 0.001 W and 0.1 W respectively.

However, this approach has limitations in that the transmission power ofthe femtocell base station will be capped regardless of whether thereare any mobile devices near to the femtocell base station that arecommunicating with a macrocell base station and that need protecting.This cap can lead to the data throughput for mobile devicescommunicating with the femtocell base station being unnecessarilyrestricted.

Therefore, there is a need for an improved approach for setting themaximum permitted transmission power for downlink transmissions frombase stations.

SUMMARY

Therefore, according to a first aspect of the invention, there isprovided a method of operating a base station, the method comprisingdetermining whether there are any mobile devices that are not associatedwith the base station that require protection from interference causedby downlink transmissions of the base station; and setting a maximumpermitted transmission power for the base station based on the result ofthe step of determining.

According to a second aspect of the invention, there is provided a basestation for use in a communication network, the base station beingconfigured to determine whether there are any mobile devices that arenot associated with the base station that require protection frominterference caused by downlink transmissions of the base station; andto set a maximum permitted transmission power for the base station basedon the result of the determination.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in detail, by way of example only,with reference to the following drawings, in which:

FIG. 1 shows an exemplary communication network;

FIG. 2 is a flow chart illustrating a method in accordance with theinvention;

FIG. 3 is a flow chart illustrating a method in accordance with theinvention in more detail;

FIGS. 4( a) and 4(b) are graphs illustrating the autocorrelationfunction for time domain reference signals with low and high signal tonoise ratios respectively;

FIG. 5 is a graph illustrating a plot of autocorrelation function peaksagainst signal to noise ratio;

FIG. 6 is a graph illustrating a plot of peak to average power ratiosagainst signal to noise ratio;

FIG. 7 is a graph illustrating a plot of autocorrelation function peaksagainst signal to noise ratio in which the scatter has been reduced;

FIG. 8 is a flow chart illustrating a method of estimating a signalquality of a reference signal in an uplink;

FIG. 9 is a graph illustrating the change in throughput on a macrocelldownlink against femtocell base station density in a macrocell sector;

FIG. 10 is a graph illustrating the change in throughput on a macrocelldownlink against femtocell base station density for a user equipment atthe edge of the macrocell;

FIG. 11 is a graph illustrating the change in throughput on a femtocelldownlink against femtocell base station density; and

FIG. 12 is a graph illustrating the change in throughput on a femtocelldownlink against femtocell base station density for a user equipment atthe edge of the femtocell.

DETAILED DESCRIPTION

Although the invention will be described below with reference to an LTEcommunication network and femtocell base stations or HeNBs, it will beappreciated that the invention is applicable to other types of third orsubsequent generation network in which femtocell base stations (whetherfor home or business use), or their equivalents in those networks, canbe deployed. Moreover, although in the embodiments below the femtocellbase stations and macrocell base stations use the same air interface(LTE), it will be appreciated that the invention can be used in asituation in which the macrocell and femtocell base stations use thesame or corresponding frequencies but different air interface schemes(for example the macrocell base stations could use WCDMA while thefemtocell base stations use LTE).

Furthermore, although the specific embodiment of the invention presentedbelow relates to controlling the maximum permitted transmission powerfor femtocell base stations, it will be appreciated by those skilled inthe art that the invention can be applied to the control of the maximumpermitted transmission power for non-femtocell base stations, such asmacrocell base stations.

FIG. 1 shows part of an exemplary communication network 2 in which theinvention can be implemented. The communication network 2 includes aplurality of macrocell base stations 4 (only one of which is shown inFIG. 1) that each define a respective coverage area—indicated bymacrocell 6. In an LTE communication network, the macrocell basestations 4 are referred to as evolved Node Bs (eNBs).

One or more femtocell base stations 8 (Home eNBs—HeNBs) can be locatedwithin the coverage area 6 of the macrocell base station 4 (althoughonly one femtocell base station 8 is shown in FIG. 1), with eachfemtocell base station 8 defining a respective coverage area—indicatedby femtocell 10.

It will be appreciated that FIG. 1 has not been drawn to scale, and thatin most real-world implementations the coverage area 10 of the femtocellbase station 8 will be significantly smaller than the coverage area 6 ofthe macrocell base station 4.

A number of mobile devices (UEs) 12 are also located in thecommunication network 2 within the coverage area 6 of the macrocell basestation 4.

Four mobile devices 12 a, 12 b, 12 c and 12 d are each associated withthe macrocell base station 4, meaning that they transmit and/or receivecontrol signalling and/or data using the macrocell base station 4. Itwill be noted that although the mobile device 12 d is also within thecoverage area 10 of the femtocell base station 8, it is associated withthe macrocell base station 4 (this could be due to the signal strengthof the macrocell base station 4 being significantly better for mobiledevice 12 d than the signal strength of the femtocell base station 8 orthe femtocell base station 8 could be restricted to specific subscribersthat don't include mobile device 12 d, etc.). Mobile devices 12 a, 12 b,12 c and 12 d are referred to collectively herein as “macro-UEs”, asthey are the mobile devices/user equipments (UEs) associated with themacrocell base station 4.

Two further mobile devices, 12 e and 12 f, are located within thecoverage area 10 of the femtocell base station 8 and are currentlyassociated with the femtocell base station 8, meaning that they transmitand/or receive control signalling and/or data using the femtocell basestation 8. Mobile devices 12 e and 12 f are referred to collectivelyherein as “femto-UEs”, as they are the mobile devices/user equipments(UEs) associated with the femtocell base station 8.

As described above, it is necessary to ensure that the downlinktransmissions from the femtocell base station 8 to the femto-UEs 12 eand 12 f do not prevent nearby macro-UEs (such as macro-UE 12 d) frombeing able to successfully receive downlink transmissions from themacrocell base station 4. A similar requirement exists for a mobiledevice that is associated with another femtocell base station, in thatthe downlink transmissions from the femtocell base station 8 to thefemto-UEs 12 e and 12 f should not prevent those mobile devices fromsuccessfully receiving the downlink transmissions from their femtocellbase station.

As described above, this problem is addressed in conventional networksby applying a cap to the transmission power used by femtocell basestations 8 to transmit signals to femto-UEs. This cap is set to a valuethat prevents these downlink signals from causing an undesirable levelof interference to mobile devices that are not associated with thefemtocell base station 8 that are in or near the coverage area 10 of thefemtocell base station 8 (such as mobile device 12 d in FIG. 1). Thiscap is applied to the transmission power regardless of whether there areany mobile devices in or near the coverage area 10 of the femtocell basestation 8 (so it would be applied, for example, even if mobile device 12d was not present).

However, in accordance with the invention (as illustrated in FIG. 2), itis determined whether there are any mobile devices that are notassociated with the femtocell base station 8 that require protectionfrom interference caused by downlink transmissions of the femtocell basestation 8 (step 101), and the transmission power cap for the femtocellbase station 8 is set accordingly (step 103).

A more detailed method of operating a femtocell base station 8 accordingto the invention is illustrated in FIG. 3. In FIG. 3, steps 111, 113,117 and 119 correspond to the step of determining (step 101) in FIG. 2.

In the following, although the invention will be described withreference to protecting mobile device 12 d (i.e. a macro-UE) that isassociated with macrocell base station 4 from downlink transmissionsfrom the femtocell base station 8, it will be appreciated that a similarmethod can be used to protect a mobile device that is associated withanother femtocell base station.

In step 111, the femtocell base station 8 attempts to identify if thereare any macro-UEs 12 that are receiving downlink transmissions from amacrocell base station 4.

In LTE, macro-UEs 12 transmit information to the macrocell base station4 before, during or after the receipt of a downlink transmission fromthe macrocell base station 4, for example an acknowledgement (ACK/NACK)signal, a channel quality indicator (CQI), sounding signals, datasignals, etc. Therefore, the femtocell base station 8 can monitor uplinkchannel(s) used by the macro-UEs for these transmissions to determine ifthere are any mobile devices nearby that might need protecting from itsdownlink transmissions.

In step 113, it is determined whether any signals detected in step 111originate from mobile devices that are not being served by (orassociated with) the femtocell base station 8.

If the femtocell base station 8 does not detect any signals frommacro-UEs 12, then the femtocell base station 8 can assume that thereare no macro-UEs nearby that need protecting from its downlinktransmissions. In this case, in step 115, the maximum permittedtransmission power for the femtocell base station 8 can be set to a highor relatively high value, for example an upper limit for thetransmission power (such as 0.1 W in LTE). The method then returns tostep 111 and repeats periodically.

If the femtocell base station 8 does detect signals from macro-UEs 12,then the method moves to step 117 in which the femtocell base station 8estimates a quality of a detected signal. This quality can be a signalto noise ratio (SNR), a signal to noise plus interference ratio (SNIR),a signal strength, or any other measure of the quality of a transmittedsignal. In some embodiments, depending on the way in which the femtocellbase station 8 detects signals in the uplink, the femtocell base station8 may be able to distinguish signals from multiple macro-UEs 12 and canestimate the quality of each of the signals. However, in alternativeembodiments, the femtocell base station 8 may not be able to distinguishthe signals and therefore performs the estimation on the signal with thehighest quality.

In a preferred embodiment, the femtocell base station 8 identifiescharacteristics of the Zadoff-Chu reference signal and estimates thesignal to noise ratio (SNR) of this signal. This embodiment is describedin more detail below with reference to FIG. 4. It will be noted that inthis embodiment the femtocell base station 8 does not distinguishbetween signals from multiple macro-UEs 12 and therefore estimates theSNR for the signal with the highest quality.

In an alternative embodiment, the femtocell base station 8 detects anddecodes the data in the uplink and determines a quality of the datasignals. It will be appreciated by those skilled in the art thatalternative techniques can be used by the femtocell base station 8 todetermine a quality of the signals in the uplink.

The femtocell base station 8 then compares the estimated quality (or thehighest estimated quality if the femtocell base station 8 can estimatethe quality for multiple signals) with a threshold value (step 119). Ina preferred embodiment where the quality is a signal to noise ratio, thethreshold can be a value in the range of 10 dB to 30 dB.

It will be noted that a macro-UE 12 will need most protection from thedownlink transmissions of the femtocell base station 8 when it is nearto the edge of the coverage area 6 of the macrocell base station 4, asthe downlink signals received at the macro-UE 12 from the macrocell basestation 4 will be relatively weak. In this situation, the macro-UE 12will need to be transmitting its uplink signals at a relatively highpower (due to its distance from the macrocell base station 4). Byestimating a quality of the uplink signal (which will be affected by thetransmission power of the macro-UE 12 d and its proximity to thefemtocell base station 8), the femtocell base station 8 can determinewhether, and/or the extent to which, the macro-UE 12 d needs protectingfrom the downlink transmissions of the femtocell base station 8.

Therefore, if the estimated quality exceeds the threshold value then thefemtocell base station 8 assumes that the macro-UE 12 d that originatedthe signal needs significant protection from the downlink transmissionsof the femtocell base station 8, and the maximum permitted transmissionpower for the femtocell base station 8 should be set at a low orrelatively low value (step 121). For example, the maximum permittedtransmission power can be set to a lower limit for the transmissionpower (such as 0.001 W in LTE).

In one embodiment, the femtocell base station 8 sets the maximumpermitted transmission power such that, at a specified pathloss from thefemtocell base station 8 (for example 80 dB), a signal received by themacro-UE 12 d from the macrocell base station 4 meets or is estimated tomeet a specified quality level (for example a target signal tointerference plus noise ratio—SINR), as in a conventional network.

The method then returns to step 111 and repeats periodically.

If the estimated quality does not exceed the threshold value then thefemtocell base station 8 sets the maximum permitted transmission powerto an intermediate value that lies between an upper and lower limit forthe transmission power (step 123). Thus, the femtocell base station 8provides some protection for the macro-UE 12 d, while allowing downlinktransmissions from the femtocell base station 8 to be transmitted at ahigher power than conventional techniques permit. In this way, the datathroughput for femto-UEs 12 e and 12 f can be improved over theconventional technique.

In a preferred embodiment, the intermediate value for the maximumpermitted transmission power is selected based on the difference betweenthe estimated quality of the signal and the threshold value. Inparticular, the value for the maximum permitted transmission power canincrease in proportion to the difference between the estimated qualityof the signal and the threshold value (up to an upper limit, ifapplicable). In a preferred embodiment where the quality is a signal tonoise ratio, if the estimated SNR is 5 dB below the threshold value,then the maximum permitted transmit power can be set to be 5 dB abovethe low or relatively low value, subject to the upper limit on themaximum permitted transmit power.

Again, the method returns to step 111 and repeats periodically.

In an alternative implementation of the invention, steps 113 and 117 canbe combined, in that the femtocell base station 8 estimates a quality(such as the SNR) of a signal in the uplink and if the estimated qualityis above a particular threshold, then a detection of a macro-UE 12 isassumed to have been made. This threshold could be the same or differentto the threshold used in step 119.

It will be appreciated that a macro-UE 12 d may move into the vicinityof the femtocell base station 8 (i.e. into or near to the coverage area10 of the femtocell base station 8) without needing to transmit anythingto its associated macrocell base station 4 (for example if the macro-UE12 d is not receiving any downlink transmissions from the macrocell basestation 4), which means that the femtocell base station 8 will not beable to detect the macro-UE 12 d in step 111.

However, as the macro-UE 12 d may need to monitor downlink controlchannels from the macrocell base station 4 (for example a broadcastchannel—BCH, or a physical downlink control channel—PDCCH), it isnecessary to make sure that the macro-UE 12 d is able to receive thesedownlink transmissions. Although these channels are designed to berelatively robust against interference, the femtocell base station 8 maystill interfere with these channels if the transmission power issufficiently high.

Therefore, in one embodiment, the femtocell base station 8 periodicallyor intermittently sets the maximum permitted transmission power to thelower limit, in order to provide the maximum protection for anymacro-UEs 12 d in its vicinity, irrespective of whether the femtocellbase station 8 detects any signals in steps 111 and 113. For example,the femtocell base station 8 can set the maximum permitted transmissionpower to the lower limit for 100 milliseconds every 1 second. This willprovide opportunities for any macro-UEs 12 d that are not transmittingany uplink signals to listen for downlink transmissions from themacrocell base station 4.

In an alternative embodiment, the femtocell base station 8 can set themaximum permitted transmission power to the lower limit whenever thefemtocell base station 8 is transmitting signals at the same time thatthe macrocell base station 4 is transmitting control channel signals. Inparticular, the femtocell base station 8 will typically be synchronisedwith the macrocell base station 4 and the control channel signals willbe sent at predetermined times and on predetermined resource blocks(RBs), so the femtocell base station will know when the macrocell basestation 4 will be transmitting the control channel signals. For example,in LTE, some control channel signals are transmitted once every 1 ms(e.g. PFICH, PDCCH), with the first four of fourteen symbols transmittedper 1 ms carrying control channel signals. Other control channels (e.g.PBCH, PSCH) are sent less frequently and use approximately seven symbolsout of every 140 symbols and a subset of the available resource blocks.

Estimation of the Quality of an Uplink Reference Signal

As described above, in a preferred embodiment of the invention, thefemtocell base station 8 identifies characteristics of the Zadoff-Chureference signal and estimates the signal to noise ratio (SNR) of thissignal.

Unlike WCDMA networks, in LTE the characteristics of uplink referencesignals are significantly different to the characteristics of both datatransmissions and thermal noise. This method exploits differences in theautocorrelation function between a portion of the time domain referencesignal and (filtered) Gaussian noise.

For an uplink reference signal occupying a small number of frequencydomain resource blocks, it would be expected that the autocorrelationfunction with high SNR would deviate from that due to (filtered)Gaussian noise. However, even with a wideband spectrally flat referencesignal, such as 50 resource blocks (the maximum for a 10 MHz system),the autocorrelation function of a portion of the time domain referencesignal deviates from the filtered Gaussian noise case.

This is true for all the Zadoff-Chu basis sequences, although the natureof the autocorrelation function does depend on the particular Zadoff-Chubasis sequence. An example of the autocorrelation function for low andhigh SNR cases with 50 resource blocks is shown in FIGS. 4( a) and 4(b)respectively.

It can be seen in FIG. 4 that the low SNR case is dominated by theautocorrelation function of the filtered Gaussian noise, while the highSNR case is dominated by the autocorrelation function of the referencesignal.

FIG. 5 shows the results of a simulation in which the autocorrelationpeaks from a single reference signal, excluding the central tap, isplotted against the SNR. This plot was obtained over a range ofdifferent reference signal parameters, numbers of resource blocks,numbers of macro-UEs, SNRs from each macro-UE and frequency resourceassignments. The simulation also included fading effects.

Thus, it can be seen from FIG. 5 that this metric, based on theautocorrelation function, can be used to estimate or predict the SNR inmany cases. However, there are a number of points in the plot wherealthough the SNR is high, the metric remains low. This scatter to theright hand side of the plot is potentially problematic, since in thesecases nearby macro-UEs might not be protected by the femtocell basestation 8. This scatter can be due to fading as well as differencesbetween the autocorrelation functions of the different Zadoff-Chu basissequences.

An alternative class of metric for the estimation of the SNR can bebased on the statistics of the time domain waveform. One simple metricis the peak to average power ratio (PAPR). High SNR reference signalsshould have low PAPR, whereas Gaussian noise has a relatively high PAPR.

Results for this metric (in linear units) are shown in FIG. 6 and it canbe seen that there is an even larger scatter apparent in the PAPR metricthan the autocorrelation metric, and as such the PAPR metric (and othermetrics based on statistics of the power) are less attractive forestimating the SNR of the uplink reference signal.

However, it has been observed that the scattering between theautocorrelation and PAPR metrics is independent, i.e. for theproblematic points with high SNR but abnormally low autocorrelationmetric, the PAPR tends to remain low (as expected for high SNR signals).For such points, the autocorrelation metric can be adjusted (upwards).This approach can be used to reduce the scatter in the autocorrelationmetric, and therefore improve the estimation of the SNR. For example, ifthe PAPR p (in linear units) is less than 3, then a minimum value can beapplied to the metric, this minimum value being given by 400+(3−p)*50.

Two additional approaches for further reducing the scatter in theautocorrelation metric have been identified.

Firstly, as the autocorrelation peaks of the reference signals tend toreduce in magnitude with distance from the main central peak, then someshaping of the autocorrelation function can be applied. To avoid anincrease in the “false detection” rate, it is important that this isonly done for samples in the autocorrelation function which are alreadysignificantly above the noise level—and so a threshold is applied priorto applying this shaping. For example, if the metric is greater than 120and the offset from the centre tap is n then the metric can be increasedby 0.6n.

Secondly, the scatter can be reduced by obtaining results over multiplemeasurements, for example by taking the maximum metric obtained from aset of four or eight measurements.

By using all of these techniques, the scatter in the autocorrelationmetric is significantly reduced. FIG. 7 illustrates the resultingrelationship between the autocorrelation metric and the SNR.

The femtocell base station 8 can make use of the relationship betweenthe autocorrelation function and the SNR to determine the SNR of anuplink signal. A method of estimating the SNR of the Zadoff-Chureference signal is shown in more detail in FIG. 8.

Firstly, the femtocell base station 8 obtains a “rough” synchronizationto the macrocell (via a network monitor mode, or, if the standardsallow, via macrocell timing measurement reports included in mobiledevice measurements, or via the X2 interface).

This rough synchronization allows the femtocell base station 8 toestimate roughly where in time the uplink reference signals frommacro-UEs are likely to be. In nearly all cases, this is the centresymbol in the 0.5 ms uplink sub-frame.

It will be appreciated that this estimation will be subject to someerror due to propagation delay from the macrocell base station 4 and thetiming advance used by macro-UEs 12. In the case of over-the-airsynchronization, which is assumed hereafter, the error will be up to onemacrocell round-trip propagation delay, which for a cell of 5 km is 33us which is roughly half the duration of an orthogonal frequencydivision multiplexing (OFDM) symbol. The error means that signalsreceived from macro-UEs 12 may arrive earlier than expected at thefemtocell base station 8.

Therefore, in step 201 of FIG. 8, the femtocell base station 8 measuresor captures a portion of the uplink reference symbol to give a timedomain reference signal. For example, the femtocell base station 8obtains the time domain reference signal from the first 512 samples ofthe reference symbol (assuming a 10 MHz bandwidth with 1024 samples plusa cyclic prefix per OFDM symbol). Despite the timing uncertainty forover-the-air synchronization, this captured portion of the referencesymbol should only contain reference signal samples from macro-UEs 12that are near to the femtocell base station 8 (i.e. there shouldn't beany samples of data symbols).

In this step, a scheduler in the femtocell base station 8 may be used toensure that there will be no uplink transmissions from femto-UEs 12 tothe femtocell base station 8 that might interfere with this measurement.

In step 203, the femtocell base station 8 determines the autocorrelationfunction for the time domain reference signal and (filtered) Gaussiannoise.

In one implementation, the femtocell base station 8 does this bynormalizing the captured time domain signal to give unit power, with theresulting sequence being denoted r, taking the fast Fourier transform(FFT) of this sequence to give f, calculating the squared magnitude(I²+Q²) for each sample of f and taking the inverse FFT of the resultingsequence to give the autocorrelation sequence a.

As the autocorrelation sequence a determined in step 203 is symmetrical(see FIG. 4), only half of the samples in a need to be retained by thefemtocell base station 8 for further processing.

In step 205, the femtocell base station 8 takes the magnitude (or, inalternative implementations, the squared magnitude) of sequence a andthen, in step 207, adjusts or zeros the central tap (corresponding tozero time lag in the autocorrelation function).

It may also be necessary to adjust or zero the tap adjacent to thecentral tap if this tap is significantly influenced by filtering in thereceive path. Such filtering has a fixed characteristic so the decisionas to adjust or zero this tap is a design decision.

Then, in step 209, the femtocell base station 8 finds the tap with thelargest magnitude (or squared magnitude) in the remaining taps, and setsthe value of a metric m to this magnitude (or squared magnitude).

The femtocell base station 8 can then determine the signal to noiseratio of the uplink reference signal using this metric (step 211). Thevalue of the SNR for the determined metric m can be determined from therelationship shown in FIG. 5 or FIG. 7, for example using acurve-fitting technique or a look-up table.

As described above, the accuracy of the SNR estimation can be improvedby considering the PAPR of the signal, shaping the autocorrelationfunction based on the distance of the peak used to determine the metricfrom the central tap and/or the metric may be estimated from signalsreceived in multiple time slots.

Therefore, the metric m may be adjusted as a function of distance fromthe central tap for example by applying a simple linear function to themetric m determined in step 209. This linear function can be asdescribed above.

Additionally or alternatively, the metric m may be adjusted as afunction of the peak to average power ratio of the captured portion ofthe uplink reference symbol. Specifically if the PAPR is below athreshold (for example 3 in linear units) then a minimum value can beimposed on the metric (again this can be a simple linear function ofPAPR). Again, this linear function can be as described above.

Again, additionally or alternatively, the metric m or SNR may beestimated from uplink reference signals captured in multiple time slotsand, for example, the highest value of the SNR obtained from thesemeasurements can be used by the femtocell base station 8 to adjust itsmaximum permitted transmission power.

FIGS. 9 to 12 illustrate the performance benefits of the inventiondescribed above.

FIG. 9 illustrates how the data throughput on a downlink from amacrocell base station is affected by an increasing number of activefemtocell base stations within the coverage area of the macrocell basestation for both a conventional fixed power cap and the scheme accordingto the invention. In particular, it can be seen that there is anegligible difference in the data throughput between the conventionalscheme and the scheme according to the invention.

FIG. 10 illustrates how the data throughput on a downlink from amacrocell base station to cell edge (5 percentile) macro-UEs is affectedby an increasing number of active femtocell base stations within thecoverage area of the macrocell base station for a conventional schemeand a scheme according to the invention. Again, there is almost anegligible difference between the two schemes.

FIG. 11 plots the data throughput on a downlink from a femtocell basestation against the number of active femtocell base stations within thecoverage area of the macrocell base station for both a conventionalfixed power cap and the scheme according to the invention. It can beseen that the scheme according to the invention provides an approximateincrease in data throughput of 5 Mb/s regardless of the number of activefemtocell base stations, which is roughly equivalent to an improvementof 25% in the data throughput.

FIG. 12 plots the data throughput on a downlink from a femtocell basestation to cell edge (5 percentile) femto-UEs against the number ofactive femtocell base stations within the coverage area of the macrocellbase station for a conventional scheme and a scheme according to theinvention. It can be seen that for cell edge (5 percentile) femto-UEsthe scheme according to the invention provides an approximate increasein data throughput of 190 kb/s regardless of the number of activefemtocell base stations, which translates to an eight-fold increase inthe data throughput.

Therefore, these graphs indicate that the adaptation of the maximumpermitted transmission power according to the invention providesperformance benefits for femto-UEs over the conventional fixed maximumpermitted transmission power scheme, while offering the same protectionto the macrocell base station downlink.

Although the invention has been described in terms of a method ofoperating a femtocell base station, it will be appreciated that theinvention can be embodied in a base station (and particularly afemtocell base station) that comprises a processor and transceivercircuitry configured to perform the described method.

There is therefore provided an improved approach for setting the maximumpermitted transmission power for downlink transmissions from basestations.

While the invention has been illustrated and described in detail in thedrawings and foregoing description, such illustration and descriptionare to be considered illustrative or exemplary and not restrictive; theinvention is not limited to the disclosed embodiments.

Variations to the disclosed embodiments can be understood and effectedby those skilled in the art in practicing the claimed invention, from astudy of the drawings, the disclosure, and the appended claims. In theclaims, the word “comprising” does not exclude other elements or steps,and the indefinite article “a” or “an” does not exclude a plurality. Asingle processor or other unit may fulfil the functions of several itemsrecited in the claims. The mere fact that certain measures are recitedin mutually different dependent claims does not indicate that acombination of these measures cannot be used to advantage. A computerprogram may be stored/distributed on a suitable medium, such as anoptical storage medium or a solid-state medium supplied together with oras part of other hardware, but may also be distributed in other forms,such as via the Internet or other wired or wireless telecommunicationsystems. Any reference signs in the claims should not be construed aslimiting the scope.

What is claimed is:
 1. A method of operating a base station, the methodcomprising: determining whether there are any mobile devices that arenot associated with the base station that require protection frominterference caused by downlink transmissions of the base station; andsetting a maximum permitted transmission power for the base stationbased on the result of the step of determining.
 2. A method as claimedin claim 1, wherein the step of setting comprises setting the maximumpermitted transmission power to a relatively high value or an upperlimit for the maximum permitted transmission power in the event that itis determined that there are no mobile devices that require protectionfrom interference caused by downlink transmissions of the base station.3. A method as claimed in claim 1, wherein the step of setting comprisessetting the maximum permitted transmission power to a relatively lowvalue or a lower limit for the maximum permitted transmission power inthe event that it is determined that there is at least one mobile devicethat is not associated with the base station that requires protectionfrom interference caused by downlink transmissions of the base station.4. A method as claimed in claim 3, wherein the step of setting comprisessetting the maximum permitted transmission power to an intermediatevalue that is higher than the relatively low value or the lower limitfor the maximum permitted transmission power.
 5. A method as claimed inclaim 1, wherein the step of determining comprises identifying whetherthere are any mobile devices that are receiving downlink transmissionsfrom a base station other than said base station.
 6. A method as claimedin claim 5, wherein the step of identifying comprises detecting signalsin an uplink from mobile devices that are not associated with said basestation to said other base station.
 7. A method as claimed in claim 6,wherein, in the event that a signal is detected in the step ofidentifying, the step of determining further comprises estimating aquality of the detected signal.
 8. A method as claimed in claim 5,wherein the step of identifying comprises estimating a quality of asignal in an uplink from mobile devices that are not associated withsaid base station to said other base station.
 9. A method as claimed inclaim 8, wherein the step of identifying comprises estimating a qualityof a reference signal in the uplink.
 10. A method as claimed in claim 9,wherein the reference signal is a Zadoff-Chu based reference signal. 11.A method as claimed in claim 7, wherein the step of setting a maximumpermitted transmission power for said base station comprises setting themaximum permitted transmission power based on the estimated quality ofthe detected signal.
 12. A method as claimed in claim 11, wherein thestep of determining comprises comparing the estimated or actual qualityof the detected signal to a threshold, and the step of setting comprisessetting the transmission power based on the comparison.
 13. A method asclaimed in claim 12, wherein if the estimated quality is above thethreshold, the step of setting comprises setting the maximum permittedtransmission power to a relatively low value or a lower limit for themaximum permitted transmission power.
 14. A method as claimed in claim12, wherein if the estimated quality is below the threshold, the step ofsetting comprises setting the maximum permitted transmission power to anintermediate value between upper and lower limits for the maximumpermitted transmission power for said base station.
 15. A method asclaimed in claim 14, wherein the intermediate value is set based on thedifference between the estimated quality and the threshold.
 16. A methodas claimed in claim 5, wherein, in the event that no mobile devices areidentified in the step of identifying, the step of setting comprisesincreasing the maximum permitted transmission power.
 17. A method asclaimed in claim 16, wherein the step of setting further comprisesperiodically or intermittently setting the maximum permittedtransmission power to a relatively low value or a lower limit for themaximum permitted transmission power.
 18. A method as claimed in claim16, wherein the step of setting further comprises setting the maximumpermitted transmission power to a relatively low value or a lower limitfor the maximum permitted transmission power for downlink transmissionsfrom said base station that coincide with downlink control channeltransmissions from a base station other than said base station.
 19. Amethod as claimed in claim 1, wherein said base station is a femtocellbase station.
 20. A base station for use in a communication network, thebase station being configured to perform the method as claimed inclaim
 1. 21. A base station as claimed in claim 20, wherein the basestation is a femtocell base station.